“External GPUs are in the news lately, what with NVIDIA’s announcement offering macOS drivers for its Titan Xp, so we thought we’d take a second to explain what, exactly, an external GPU is — and how you’d go about getting one,” Serenity Caldwell writes for iMore.

“Like external hard drives, [external GPUs] essentially allow you to stick a GPU in a Thunderbolt housing, where you can then connect it to your computer; from there, when you run games and visual apps optimized for that GPU you should see significant performance improvements,” Caldwell writes. “Awesome, right? Well, almost.”

“Here’s the issue: Macs don’t officially support external GPUs,” Caldwell writes. “That’s not to say you can’t use an external GPU, only that Apple Support won’t bail you out if you do something that doesn’t agree with your Mac. Proceed at your own risk, here be dragons, et cetera.”

The original concept of the Mac Pro was NOT to use eGPUs. It had two quite fast GPUs (a pair of D700s) within it. The problem was that Apple was expecting developers to write software optimized for multiple GPUs, and that never happened to any significant amount. When significantly faster, single chip GPUs came around there was no Apple built in way to take advantage of those chips.

On the other hand, the latest MacBook Pro can utilize eGPUs as shown by test sites such as BareFeats. If you need an Nvidia 1080 or hotter GPU you can do it through external expansion and still have a reasonable, portable computer on the road. (Note reasonable, not top of the line performance.)

External GPUs are workarounds for mobile computers. Better than nothing, and also better sometimes than a new machine.

But even very compact PC chassis are able to internally accommodate a top GPU using faster than Thunderbolt 3.0 connection.

The thing is Apple cornered us everywhere and the opportunity appeared. But this is only getting efficient with TB 3.0, an equivalent to PCIe 8X. But current PCIe 3.0 = 32 lanes. The GPU itself wont scale directly as being 2X or more but it will suffer from a bottleneck transporting information and the delay may reduce the performance maybe 30% for a medium to high performance card. The GPU will suffer more the higher the performance it offers.

“But even very compact PC chassis are able to internally accommodate a top GPU using faster than Thunderbolt 3.0 connection.”
Not true. Many of the half towers and compact PCs won’t take the double wide GPU cards and even more won’t take the thermal load or provide the power necessary for the top GPU cards.

“The thing is Apple cornered us everywhere and the opportunity appeared. But this is only getting efficient with TB 3.0, an equivalent to PCIe 8X.”
This depends upon which version of PCIe you are referencing.

“But current PCIe 3.0 = 32 lanes. The GPU itself wont scale directly as being 2X or more but it will suffer from a bottleneck transporting information and the delay may reduce the performance maybe 30% for a medium to high performance card. The GPU will suffer more the higher the performance it offers.”
I suggest you go over the the BareFeats site (http://barefeats.com/bandwidth.html) and see how its tests negate your premise. There are some cases where GPUs under TB 3 are I/O constrained, but not many.
Also, PCIe 1.0 spec supported 32 lanes! I don’t believe anyone shipped any equipment with over 16 lanes, but the spec included that capability. Further, I don’t expect anyone to ship 32 lane implementations of PCIe 3.0 as there is very little real need for 31.5 GB/s today (or 4.0, which is expected to start shipping in products in 2017, or need for 63 GB/s in the near future).